This invention relates to circuit breaker assemblies having an improved latching system that substantially decreases mechanical trip time. The improved latching system can be utilized, but not limited to circuit breaker assemblies rated for residential and lower current industrial applications and for high ampere-rated circuit breaker assemblies.
Conventional circuit breaker assemblies utilize a thermal-magnetic trip unit to automatically sense overcurrent circuit conditions and to subsequently interrupt circuit current accordingly. It is the practice of the circuit protection industry to mount a magnet portion of the magnetic trip unit around a bimetal trip unit and to arrange an armature as part of the circuit breaker latching system. It is well appreciated in the electric circuit protection field that the latching surfaces within the circuit breakers latching system must be carefully machined and lubricated in order to ensure repeated latching and unlatching between the surfaces over long periods of continuous use.
The special machining that is required includes a time consuming polishing process or a special machining or shaving operation on the latch systems latch surfaces. The smooth low friction surfaces are required to minimize the amount of tripping force that must be applied to overcome the bias of the operating spring and the static friction on the contracting latch surfaces. The trip force is the amount of force that must be applied to the trip bar to overcome the latch spring bias and latch surface friction
In operation, a magnetic trip unit comprising an armature and a magnet is actuated upon the occurrence of an overcurrent condition. The actuation causes the armature, which is biased away from the magnet by a spring, to be rapidly driven towards the magnet so that a trip bar is activated. The thermal trip unit comprising a bimetal element senses overcurrent conditions by responding to the temperature rise on the bimetal element. When an overcurrent condition occurs over a period of time, the bimetal flexes and activates the trip bar.
Once activated, the trip bar sets in motion the activation and disengagement of a latching system comprising a primary latch, secondary latch, and a cradle. The trip bar, secured to the secondary latch, drives the secondary latch clockwise about a fixed point so that the secondary latch is moved out of contact with the primary latch. The primary latch in turn is positioned to prevent the rotation of the cradle. When the primary latch is released from the secondary latch, the cradle acts on the primary latch urging it to rotate clockwise about a fixed point. Once the primary latch is moved out of contact with the cradle, the cradle is released allowing it to rotate counterclockwise about a fixed point. As the cradle pivots the upper and lower links collapse under the biasing of an operating spring to draw a moveable contact arm containing a moveable contact to the open position. In the open position the moveable contact and a fixed contact are separated thereby terminating the circuit.
The primary latch and the secondary latch have a plurality of latching surfaces. The latching surfaces are defined as the surface of the latch that makes physical contact with any adjoining surface. The first latching surface of the secondary latch is positioned against the second latching surface of the primary latch. A first latching surface of the primary latch is positioned against the latching surface of the cradle. As previously described when the trip bar is actuated, it drives the secondary latch so that the secondary latch rotates about its pivot causing the first latching surface of the secondary latch to break contact with the second latching surface of the primary latch. Once this occurs, the first latching surface of the primary latch has a force bearing on it by the cradle at the cradle latching surface. If this force is great enough to overcome any resistant forces existing between the latching surfaces, the primary latch will rotate about its pivot point so that the first latching surface of the primary latch breaks contact with the latching surface of the cradle. Once released, the cradle rotates counterclockwise and set in motion a chain of events that trips the breaker.
Conventionally both the cradle and the primary latch are fabricated from a stamping operation followed by a shaving operation to flatten and smooth the latching surface of the cradle and the latching surfaces on the primary latch to maintain a low trip force between the cradle and the primary latch. To aid in the release of the latches there is a primary latching force provided by the operating spring. During use there is often a degradation of the latching surfaces due to wear and contaminates on the various latching surfaces. Even when the latching surfaces are prepared in an effort to minimize friction and the various springs provide a biasing force it is unpredictable if and when the latching system will be fully activated. If significant contaminates or excessive wear exists on the various latching surfaces, the latching system will not activate and result in a stalled situation between the cradle and the primary latch. In particular, once the primary latch is released by the secondary latch, the cradle through the latching surface of the cradle and supplied by provides a force on the primary latch at the first latching surface. This force must be great enough to overcome the friction forces acting between the first latching surface of the primary latch and the latching surface of the cradle. If contaminants or other sources cause the friction between these latching surfaces to become too large the first latching surface of the primary latch will not rotate and release the cradle so that the system is in a stalled situation.
Conventional circuit breakers have a size limitation imposed upon them in order to fit into panel boards of residential, office and light industrial applications. While the outer dimensions of the circuit breaker are fixed, short circuit current magnitudes available from electrical utilities have increased, requiring circuit breaker designers to seek new and improved operating and trip mechanisms which limit the energy let-through. To do this, one must minimize the current and/or the time from the onset of overload to arc extinction. One way to accomplish this is to provide an extremely fast acting circuit breaker capable of early contact separation upon detection of an overload.